This invention relates broadly to an improvement in the production of containers for nuclear fuel for service in nuclear fission reactors, and of nuclear fuel elements comprising the containers having therein a body of nuclear fuel materials such as compounds of uranium, plutonium and thorium, and the products thereof comprising nuclear fuel containers and elements.
Nuclear reactors are presently being designed, constructed and operated in which the nuclear fuel is contained in fuel elements which can have various geometric shapes, such as plates, tubes, or rods. The fuel material is usually enclosed in a corrosion-resistant non-reactive, heat conductive container or sheath. The elements are assembled together in a lattice at fixed distances from each other in a coolant flow channel or region forming a fuel assembly, and sufficient fuel assemblies are combined to form the nuclear fission chain reacting assembly or reactor core capable of a self-sustained fission reaction. The core in turn is enclosed within a reactor vessel through which a coolant is passed.
The container serves several purposes and two primary purposes are: first, to prevent contact and chemical reactions between the nuclear fuel and the coolant or the moderator if a moderator is present, or both if both the coolant and the moderator are present; and second, to prevent the radioactive fission products, some of which are gases, from being released from the fuel into the coolant or the moderator, or both if both the coolant and the moderator are present. Common container materials are stainless steel, aluminum and its alloys, zirconium and its alloys, niobium (columbium), certain magnesium alloys, and others. The failure of the container, i.e., a loss of the leak tightness, can contaminate the coolant or moderator and the associated systems with radioactive long-lived products to a degree which interferes with plant operation.
Problems have been encountered in the manufacture and in the operation of nuclear fuel elements which employ certain metals and alloys as the container material due to mechanical or chemical reactions of these container materials under certain circumstances. Zirconium and its alloys, under normal circumstances, are excellent nuclear fuel containers since they have low neutron absorption cross sections and at temperatures below about 750.degree. F. (about 398.degree. C.), are strong, ductile, extremely stable and non-reactive in the presence of demineralized water or steam which are commonly used as reactor coolants and moderators.
However, fuel element performance has revealed a problem with the brittle splitting of the container due to the combined interactions between the nuclear fuel, the container and the fission products produced during nuclear fission reactions. It has been discovered that this undesirable performance is promoted by localized mechanical stresses due to fuel-container differential expansion (stresses in the container are localized at cracks in the nuclear fuel). Corrosive fission products are released from the nuclear fuel and are present at the intersection of the fuel cracks with the container surface. Fission products are created in the nuclear fuel during the fission chain reaction during operation of a nuclear reactor. The localized stress is exaggerated by high friction between the fuel and the container.
Within the confines of a sealed fuel element, hydrogen gas can be generated by the slow reaction between the container, and the residual water inside the container may build up to levels which under certain conditions can result in localized hydriding of the container with concurrent local deterioration in the mechanical properties of the container. The container is also adversely affected by such gases as oxygen, nitrogen, carbon monoxide and carbon dioxide over a wide range of temperatures.
The zirconium container of a nuclear fuel element is exposed to one or more of the gases listed above and fission products during irradiation in a nuclear reactor and this occurs in spite of the fact that these gases and fission product elements may not be present in the reactor coolant or moderator, and further may have been excluded as far as possible from the ambient atmosphere during manufacture of the container and the fuel element. Sintered refractory and ceramic compositions, such as uranium dioxide and other compositions used as nuclear fuel, release measurable quantities of the aforementioned gases upon heating, such as during fuel element manufacture and further release fission products during irradiation. Particulate refractory and ceramic compositions, such as uranium dioxide powder and other powders used as nuclear fuel, have been known to release even larger quantities of the aforementioned gases during irradiation. These released gases are capable of reacting with the zirconium container containing the nuclear fuel.
Thus in light of the foregoing, it has been found desirable to minimize attack of the container from water, water vapor, hydrogen and other gases reactive with the container from inside the fuel element throughout the time the fuel element is used in the operation of nuclear power plants. One such approach has been to find materials which will chemically react rapidly with the water, water vapor, hydrogen, and other gases to eliminate these from the interior of the container, and such materials are called getters.
A number of other approaches to this problem have been enumerated in some detail in U.S. Pat. No. 4,022,662, issued to Gordon and Cowan May 10, 1977; U.S. Pat. No. 4,029,545 issued to Gordon and Cowan June 14, 1977; and U.S. Pat. No. 4,045,288, issued to Armijo Aug. 30, 1977, all assigned to the same assignee of this application for patent. The contents of the disclosures of said U.S. Pat. Nos. 4,022,662, 4,029,545 and 4,045,288 are accordingly incorporated by reference in this application for patent.
For example, it has been suggested to introduce a barrier between the nuclear fuel material and the container casing holding the nuclear fuel material as disclosed in U.S. Pat. No. 3,230,150 (copper foil), German Patent Publication DAS No. 1,238,115 (titanium layer), U.S. Pat. No. 3,212,988 (sheath of zirconium, aluminum or beryllium), U.S. Pat. No. 3,018,238 (barrier of crystalline carbon between the UO.sub.2 and the zirconium cladding), and U.S. Pat. No. 3,088,893 (stainless steel foil). While the barrier concept proves promising, some of the foregoing references involve incompatible materials with either the nuclear fuel (e.g., carbon can combine with oxygen from the nuclear fuel), or the container (e.g., copper and other metals can react with the container altering the properties of the container), or the nuclear fission reaction (e.g., by acting as neutron absorbers). None of the listed references disclose solutions to the recently discovered problem of localized chemical mechanical interactions between the nuclear fuel and the container.
Further approaches to the barrier concept are disclosed in U.S. Pat. No. 3,969,186, issued July 13, 1976 (refractory metal such as molybdenum, tungsten, rhenium, niobium and alloys thereof in the form of a tube or foil of single or multiple layers or a coating on the internal surface of the container and U.S. Pat. No. 3,925,151, issued Dec. 9, 1975 (liner of zirconium, niobium or alloys thereof between the nuclear fuel and the containers with a coating of a high lubricity material between the liner and the container).
A recently proposed response to the problem of fuel container or element failures attributable to deleterious interactions between container casings composed of zirconium or zirconium alloys and the nuclear fuel and/or the fission products thereof, has been to provide a copper metal lining within the casing as a barrier layer on the inside surface of such fuel casings whereby it will be interposed between the zirconium casing and the nuclear fuel provided therein. A layer of copper plating is generally considered to primarily function as a physical and chemical barrier impeding destructive fission products, such as cadmium, cesium, iodine and the like, from contacting and attacking the zirconium or zirconium alloy of the fuel element casing or container.
The copper layer or intermediate barrier thereof provides a preferential reaction site or body for reaction with volatile impurities or fission products from within the nuclear fuel element, and thus serves to protect or shield the fuel container casing from exposure to and attack by such destructive agents.
However, the fission reactions occurring in nuclear reactors subject the internal surface portion of a fuel container, such as an inner layer of copper cladding or plating, to temperatures that are discernably greater than the temperatures reached by the outer portion of a fuel container such as the zirconium or alloy thereof of the fuel casing that is continuously in contact with the circulating coolant or heat transfer medium. Exposure over prolonged periods to reactor fission temperatures and environments tend to promote interdiffusions of copper from the inner plating layer and the zirconium or its alloy of the outer casing substrate at their contacting interface. Any significant interdiffusion of these contacting metals or their alloys can result in the formation of low melting (for example below about 1200.degree. C.) liquid entectic phases between the zirconium or its alloy of the container casing and the lining of copper cladding, as well as the formation of intermetallic phases of inferior or inadequate properties such as reduced resiliency and tensile strength or embrittlement.
Measures for deterring the occurrence of such interdiffusions of the adjoining different metals or alloy of the container casing and the internal liner with the accompanying debilitating effects have been proposed. One remedial measure for precluding the occurence of this metal interdiffusion phenomenon and the property losses attributable thereto comprises deploying a diffusion barrier intermediate the container casing substrate and the overlying liner within the casing. An oxide of zirconium or alloy thereof has been found to be an effective diffusion barrier, whereby simply oxidizing the inner surface of a zirconium or alloy thereof container casing comprising the substrate for deposition of the copper liner thereover provides a feasible and advantageous means for preventing interdiffusion of the metals and the accompanying weakening effects attributable thereto. Oxidation of zirconium or alloys thereof can be readily achieved by the application thereto of steam.
Although an oxide phase at the interface of the adjoining metals provides an apt remedy for the interdiffusion problem, the presence of an oxide layer covering the substrate surface of the zirconium casing imposes significant restrictions upon the techniques or options for effective application of a copper layer thereover because such oxides lack effective electrical conductance. For instance, the highly effective, common electrolytic deposition procedures for metal, such as disclosed in U.S. Pat. No. 4,017,368 and 4,137,131, would be decidedly curtailed or precluded from use by the interposing of an oxide phase of very low electrical conductivity between the metal substrate and metal-containing electrolytic solution, whereby less effective or complex alternatives must be utilized, such as the electroless deposition procedure disclosed in U.S. Pat. No. 4,093,756. Moreover, it appears that the depositing of copper metal on an oxide surface with such electroless depositions procedure is prone to a blistering phenomenon in the deposited layer or cladding.